Physicists entangle 15 trillion hot atoms

Abstract illustration of quantum entanglement. — (Image credit: Shutterstock)

Physicists set a new record by linking together a hot soup of 15 trillion atoms in a bizarre phenomenon called quantum entanglement. The finding could be a major breakthrough for creating more accurate sensors to detect ripples in space-time called gravitational waves or even the elusive dark matter thought to pervade the universe.

Entanglement, a quantum phenomena Albert Einstein famously described as "spooky action at a distance," is a process in which two or more particles become linked and any action performed on one instantaneously affects the others regardless of how far apart they are. Entanglement lies at the heart of many emerging technologies, such as quantum computing and cryptography.

Entangled states are infamous for being fragile; their quantum links can be easily broken by the slightest internal vibration or interference from the outside world. For this reason, scientists attempt to reach the coldest temperatures possible in experiments to entangle jittery atoms; the lower the temperature, the less likely atoms are to bounce into each other and break their coherence. For the new study, researchers at the Institute of Photonic Science (ICFO) in Barcelona, Spain, took the opposite approach, heating atoms to millions of times hotter than a typical quantum experiment to see if entanglement could persist in a hot and chaotic environment.

"Entanglement is one of the most remarkable quantum technologies, but it is famously fragile," said Jia Kong, a visiting scientist at ICFO and lead author of the study. "Most entanglement-related quantum technology has to be applied in a low-temperature environment, such as a cold atomic system. This limits the application of entanglement states. [Whether or not] entanglement can survive in a hot and messy environment is an interesting question."

The researchers heated a small glass tube filled with vaporized rubidium and inert nitrogen gas to 350 degrees Fahrenheit (177 degrees Celsius), coincidentally the perfect temperature to bake cookies. At this temperature, the hot cloud of rubidium atoms is in a state of chaos, with thousands of atomic collisions taking place every second. Like billiard balls, the atoms bounce off each other, transferring their energy and spin. But unlike classical billiards, this spin does not represent the physical motion of the atoms.

In quantum mechanics, spin is a fundamental property of particles, just like mass or electric charge, that gives particles an intrinsic angular momentum. In many ways, the spin of a particle is analogous to a spinning planet, having both angular momentum and creating a weak magnetic field, called a magnetic moment. But in the wacky world of quantum mechanics, classical analogies fall apart. The very notion that particles like protons or electrons are rotating solid objects of size and shape doesn't fit the quantum worldview. And when scientists try to measure a particle's spin, they get one of two answers: up or down. There are no in-betweens in quantum mechanics.

The researchers shot a beam of polarized light at the tube of rubidium atoms. Because the atoms' spins act like tiny magnets, the polarization of the light rotates as it passes through the gas and interacts with its magnetic field. This light-atom interaction creates large-scale entanglement between the atoms and the gas. When researchers measure the rotation of the light waves that come out the other side of the glass tube, they can determine the total spin of the gas of atoms, which consequently transfers the entanglement onto the atoms and leaves them in an entangled state.

In fact, the "hot and messy" environment inside the glass tube was key to the experiment's success. The atoms were in what physicists call a macroscopic spin singlet state, a collection of pairs of entangled particles' total spin sums to zero. The initially entangled atoms pass their entanglement to each other via collisions in a game of quantum tag, exchanging their spins but keeping the total spin at zero, and allowing the collective entanglement state to persist for at least a millisecond. For instance, particle A is entangled with particle B, but when particle B hits particle C, it links both particles with particle C, and so on.

However, their results may help to develop ultra-sensitive magnetic field detectors, capable of measuring magnetic fields more than 10 billion times weaker than Earth's magnetic field. Such powerful magnetometers have applications in many fields of science. For example, in the study of neuroscience, magnetoencephalography is used to take images of the brain by detecting the ultra-faint magnetic signals given off by brain activity.

Things get hot and messy